Search

The room-temperature reaction of lithium hydroxide with aluminum hydroxide (bayerite) in the ratio of 1:2 produces lithium dialuminate, LiAl[sub 2](OH)[sub 7][center dot]2H[sub 2]O. Rietveld refinement of x-ray and neutron powder diffraction patterns indicate that the structure is composed of [LiAl[sub 2]-(OH)[sub 6]][sup +] layers separated by water molecules and hydroxide ions. Disorder in the stacking sequence leads to indexing the diffraction pattern on a hexagonal cell (a = 5.0972(3) [angstrom], c = 7.5524(9) [angstrom] [times] 3n layers); however, the structure is better described by a C-centered monoclinic cell with the contraints the b = a[radical]3 and cos [beta] = -a/3c (a = 5.097 [angstrom], b = 8.829 [angstrom], c = 7.741 [angstrom], and [beta] = 103.0[degree]). The diffraction patterns were refined using a model of up to 54 layers ([approx]400 [angstrom]). The layer stacking vector, which defines the angle between layers, is less than expected based on the position of the diffraction peaks. The structural complexities present in lithium dialuminate have their origins in the conditions of synthesis and the layered nature of the material. 29 refs., 10 figs., 4 tabs.

The anisotropic susceptibility of YBa 2 Cu 3 O x (6.46⪕ x ⪕6.98) has been measured as a function of oxygen composition. Samples were prepared by a high temperature zirconium-getter annealing technique followed by magnetic orientation in epoxy. The normal state magnetic susceptibility was found to be weakly dependent on temperature. The room temperature values of the susceptibility decrease linearly as the oxygen composition decreases, whereas the difference in susceptibilities for magnetic fields parallel and perpendicular to the c -axis is nearly constant. This suggests that in this range of oxygen composition the susceptibility anisotropy comes mostly from the anisotropic part of the Van Vleck susceptibility and the spin susceptibility remains nearly isotropic. Neutron diffraction results show a significant oxygen occupancy at the O(5) site. Nuclear quadrupolar and magnetic resonance spectra of the plane copper indicate a marked degree of electronic disorder, confirming that there are microscopic structural effects from the high temperature zirconium-getter annealing procedure.

The synthesis of layered cuprates with nonmagnetic and fixed oxidation state cations separating CuO planes has, until now, led to nonsuperconducting materials. Recently we were able to introduce superconductivity in YSrCuGaO system by appropriate doping and annealing.

The 63 Cu NMR spin-lattice relaxation rate of YBa 2 Cu 3 O 6.98 (T C =93.3K) at the plane site shows classic Korringa temperature dependence in the normal state with T 1 T=230±20 Kms. This resuit is consistent with Fermi quasi-particle character of the low lying electronic excitations near the Fermi energy. Precursory behavior is observed just above T c in the relaxation data. This may be consistent with order parameter fluctuations.

Using a resonant‐cavity perturbation technique, the microwave absorption at 16.8 GHz of the high‐Tc compound YBa2Cu3O7 is measured as a function of temperature (4–300 K) and magnetic field (0–7 T). At zero field the microwave loss shows a rapid decrease below 93 K, the onset of the superconducting transition. An important absorption, which is a decreasing function of temperature, is observed in the superconducting state.When a magnetic field is applied, the transition is broadened and shifted toward lower temperatures: an upper‐critical field Hc2(T=0)∼ 150 T is obtained via extrapolation of the data. In the superconducting state, the absorption is found to be highly dependent on the magnetic‐field intensity. For magnetic fields above 500 Oe, the absorption is a slowly increasing function and it is understood in terms of the temperature behavior of Hc2 ; for H

Polycrystalline samples of ${\mathrm{La}}_{1.83}$${\mathrm{Sr}}_{0.17}$Cu${\mathrm{O}}_{4}$ were studied by pulsed nuclear-magnetic-resonance methods on the copper nucleus over a temperature range which included both normal and superconducting phases. A Korringa constant 14\ifmmode\pm\else\textpm\fi{}1 K sec and a minimum value for the energy gap $\frac{2\ensuremath{\Delta}}{{k}_{B}{T}_{c}}=1.3\ifmmode\pm\else\textpm\fi{}0.2$ were found. A comparison of these results with the larger gap reported from infrared absorption and from tunneling experiments indicates that there is substantial anisotropy in the pairing interaction. Homogeneous line narrowing in the superconducting state was observed.

We investigate here the anisotropic magnetic and electric properties of aligned crystalline grains of YBa2Cu3O7−x. The microwave absorption is indeed consistent with the expected temperature behaviors of the resistivity in the normal state along the c axis and in the a‐b plane. In the superconducting state, both the absorptions in pure rf electric and magnetic fields are high compared to the conventional superconductors; for the magnetic case activation energies are deduced and the dispersion is related to the diamagnetic susceptibility. These results are discussed qualitatively in relation to similar data obtained on single crystals and ceramic samples.

Trigonal ordering is evident from electron microscopy of quenched powders of composition YBa 2 Cu 3 O 6.5 . The powders are a mixture of phases, some of which are based upon two trigonal phases with a = 0.54 and c = 0.66 nm. In the first the yttrium and barium atoms order on the (111) planes of the parent perovskite structure rather than the (001) while in the second stacking sequence the yttrium layer of this structure is interchanged with one of the copper layers.

Le compose du titre cristallise dans le systeme quadratique avec le groupe d'espace P4 2 /n. Affinement de la structure jusqu'a 0,0254

High-${T}_{c}$ superconductivity is investigated in the Y-Ba-Cu-O system. Regions of possible compound formation are ${\mathrm{Y}}_{2\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Ba}}_{\mathrm{x}}$${\mathrm{CuO}}_{4\mathrm{\ensuremath{-}}\mathrm{x}/2+\mathrm{\ensuremath{\delta}}}$ and ${\mathrm{Y}}_{2\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Ba}}_{1+\mathrm{x}}$${\mathrm{Cu}}_{2}$${\mathrm{O}}_{6\mathrm{\ensuremath{-}}\mathrm{x}/2+\mathrm{\ensuremath{\delta}}}$. ${T}_{c}$ values g90 K are found for the former with x=0.8 (the Chu material) and for the latter with x=0.2 and may imply a common superconductivity phase in both samples. Critical fields determined by resistance and low-field ${d}_{c}$ susceptibility measurements are reported.